Due to depletion of petrochemical fuels, development of alternative energy sources has become a hot issue in recent years. As an alternative energy source, ethanol can be produced from cellulosic biomass, which is a renewable and most abundant carbon source stored on earth. Researches have been focused on effective decomposition of cellulose, hemi-cellulose and lignin present in lignocellulosic biomass through screening of novel strains with cellulolytic property as well as improvement of enzymatic saccharification and fermentation process.
Major steps involved in the cellulosic fuel production include i) the enzymatic saccharification of plant biomass into simple carbohydrates carried out by the synergistic action of at least three enzymes (e.g., endoglucanase, exoglucanase and β-glucosidase) and ii) microbial fermentation of these carbohydrates into value-added fuels. Recently, many researches are focused on simultaneous saccharification and fermentation (SSF), which combines enzymatic saccharification and microbial fermentation process in a same reactor, significantly enhancing the efficiency of ethanol production by reducing inhibitory action of the saccharifying enzymes and equipment costs. As enzymatic saccharification is one of the most expensive steps in the overall process, researchers have endeavored to enhance the activity of the enzymes used in the saccharification or to develop novel strains capable of producing these enzymes. Advanced genetic engineering has enabled production of strains for simultaneous saccharification and fermentation by the introduction of genes encoding saccharifying enzymes into the fermentation strain or vice versa. The saccharifying enzyme as a heterogenous gene, however, shows considerably low level of expression, and it also has negative effects on cell growth and metabolism when overexpressed. Therefore, modifying the regulation of the endogenous pathway is considered more advantageous than the use of heterogenous genes.
E. coli is one of the most effective microorganisms for lignocellulosic fuel production because of its ability to utilize all sugars derived from hydrolysis of biomass. However, the potential of E. coli is limited due to carbon catabolite repression (CCR), i.e., inhibition of biosynthesis of enzymes involved in catabolism of carbon sources other than the preferred one (e.g., glucose) in hydrolysate. Thus, sugars such as xylose and arabinose cannot be metabolized until the depletion of glucose. This preference toward glucose utilization impedes fermentation process by reducing the productivity and affects downstream processes due to unused carbon sources. Composition of sugar mixture obtained from lignocellulosic hydrolysate may vary, but glucose and xylose would occupy a significant portion of them. Therefore, there is still need of developing a mutant E. coli with enhanced sugar utilization and capable of utilizing sugars simultaneously without preference in order to improve the cost, efficiency and usability in the cellulosic fuel production.